A DC rotating electrical machine typically includes an armature winding mounted on the rotor surrounded by a field system mounted on the stator. A rotor-connected commutator with copper or copper alloy segments and stationary brushgear are used to control the commutation of current in the rotor winding based on the angular position of the rotor. In a further development of the DC rotating electrical machine, the armature winding is mounted on the stator and the field system is mounted on the rotor. An electronic switching circuit is used to control the commutation of current in the armature winding in relation to the angular position of the rotor. The following description is based on a DC rotating electrical machine having this construction.
The armature winding will include a plurality of coils that are located in winding slots formed in a surface of the armature assembly. The rotor provides a rotating magnetic field and this can be generated by permanent magnets, superconducting windings with a suitable excitation power supply or conventional windings with slip rings or brushless excitation power supply. Electrical machines using electronic commutation may operate at much higher voltages (voltages of several kV are possible) and proportionally lower currents than conventional electrical machines that use brushes.
DC rotating electrical machines employing electronic commutation may be used as generators for wind turbine applications. A turbine blade assembly may be used to drive the rotor of the generator, either directly or by means of a gearbox. If the rotor is connected directly to the turbine blade assembly then the generator will run at very low speeds and, in order to minimise the amount of ineffective copper in the endwindings of the coils, to minimise the amount of magnetic steel that is required and to simplify construction, the generator will normally have a very large number of poles (typically 50 or more for the largest generators) and a very small number of slots per pole-pair.
An armature winding with n coils per pole-pair would require only n commutating events during the time taken for the relative movement between the armature and the field system of one pole-pair. For simplicity this is referred to as the number of commutating events per pole-pair. The number of commutating events per pole-pair in a conventional DC rotating electrical machine is equal to the number of coils per pole-pair and n is not necessarily an integer.
The purpose of the present invention is to provide an improved armature winding that allows a much larger number of commutation events per pole-pair than the number of coils per pole-pair.